I had a problem like this crop up today myself and had I not seen this note about it being a USPS module for shipping, it occurred to me that I had not reinstalled the Canada Post module, and that turned out to be the fix for the solution as well. So it doesn't necessarily need to be USPS, if you have any module that deals with the shopping_cart.php file, it's worth checking it to be sure you can resolve it.
Either or, I'm just glad to have it fixed now,
All the sites I maintain now are finally caught up to 1.37.
Marian



